1. Field of the Invention
This invention relates to a zoom lens and an optical apparatus using the same, and particularly is suitable for an optical apparatus such as a photographic camera, a video camera, a digital camera or a camera for broadcasting using a diffraction optical element in a portion of the lens system thereof to thereby well correct aberrations, particularly chromatic aberration.
2. Related Background Art
In optical apparatus such as electronic still cameras, video cameras and photographic cameras, with the downsizing and lighter weight thereof, the downsizing and lighter weight of image pickup zoom lenses used therein have heretofore been strongly demanded.
As a zoom lens, it has been demanded to have a high variable power ratio, and to have high optical performance in which aberration correction has been effected well with the higher definition of an image pickup element.
As a method of effecting aberration correction well, it is known that it is effective to effect it by the use of an aspherical surface. The use of an aspherical surface is effective to correct aberrations such as spherical aberration, curvature of image field and coma. The correction of chromatic aberration, however, is difficult. Particularly in a compact zoom lens with a zooming ratio of about three having a small number of lenses, the combination of glass materials is limited because the number of lenses is small, and it is difficult to correct chromatic aberration well. Of the chromatic aberration, it is particularly difficult to eliminate secondary spectrum. In the latest zoom lenses, it is a great task to shorten the full length of the lens, and yet correct aberrations including chromatic aberration well and maintain optical performance good.
On the other hand, as a method of keeping chromatic aberration small, it has been proposed in recent years to apply a diffraction optical element to an image pickup optical system. There has also been proposed what uses a diffraction optical element in a zoom lens to aim at higher variable power ratio.
As a zoom lens of the simplest lens construction, there is a negative-positive two-unit zoom lens which comprises, in succession from the object side, a first lens unit having negative refractive power, and a second lens unit having positive refractive power, and in which both lens units are moved to thereby effect a focal length change. This negative-positive two-unit zoom lens forms a retrofocus type and has a relatively long back focal length and is therefore best suited for the optical system of a camera like a single-lens reflex camera which requires a long back focal length. What uses a diffraction optical element in such negative-positive two-unit zoom lens is proposed, for example, in Japanese Patent Application Laid-Open No. 10-161022 (corresponding U.S. Pat. No. 6,094,314), Japanese Patent Application Laid-Open No. 10-213744, etc.
As a zoom lens of the simplest lens construction having a relatively short back focal length, there is a positive-negative two-unit zoom lens which comprises, in succession from the object side, a first lens unit of positive refractive power and a second lens unit of negative refractive power and in which both lens units are moved to thereby effect a focal length change (zooming). In such positive-negative two-unit zoom lens, what uses a diffraction optical element to achieve higher performance is proposed, for example, in Japanese Patent Application Laid-Open No. 9-197273, Japanese Patent Application Laid-Open No. 9-197274, Japanese Patent Application Laid-Open No. 10-142504. Japanese Patent Application Laid-Open No. 10-142505 (U.S. Pat. No. 6,067,196 corresponding to these four cases), etc. The assignee of the application also has proposed a zoom lens of similar construction in Japanese Patent Application Laid-Open No. 11-149043.
These positive-negative two-unit zoom lenses form the so-called telephoto type. Therefore, the optical full length becomes short and the back focal length at the wide angle end is also short. Such zoom lens has only two lens units and is therefore simple in mechanical construction, and is often used in a lens shutter camera or the like which does not require a particularly long back focal length.
Generally, as an optical system (taking lens) suitable for a lens shutter camera or the like which does not require a long back focal length and in which the exit angle in the optical system does not affect very much, it is preferable for the last lens unit (the lens unit most adjacent to the image plane side) to have negative refractive power because it can form the telephoto type and shorten the optical full length. This also holds true of zoom lenses having three or more lens units.
Three-unit zoom lenses comprising three lens units and having a relatively short back focal length which use a diffraction optical element to achieve higher performance are proposed, for example, in Japanese Patent Application Laid-Open No. 11-23968 (corresponding U.S. Pat. No. 5,982,544), etc. The assignee of the application also has proposed a zoom lens of similar construction in Japanese Patent Application Laid-Open No. 11-109242.
Any of these is a positive-positive-negative three-unit zoom lens comprising, in succession from the object side, a first lens unit having positive refractive power ratio (zooming ratio), a second lens unit having positive refractive power, and a third lens unit having negative refractive power, and realizes a zoom lens of high variable power ratio (zooming ratio) in which chromatic aberration is sufficiently corrected by the diffraction optical element.
Also, four-unit zoom lenses comprising four lens units as a whole which use a diffraction optical element to achieve higher performance are proposed, for example, in Japanese Patent Application Laid-Open No. 9-211329 (corresponding U.S. Pat. No. 5,872,658), Japanese Patent Application Laid-Open No. 10-148757, Japanese Patent Application Laid-Open No. 10-160916, Japanese Patent Application Laid-Open No. 10-333036, etc. Any of these comprises a construction having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power.
The negative-positive two-unit zoom lens becomes long in the optical full length and is therefore disadvantageous for a compact optical apparatus because the entire apparatus becomes bulky.
Also, the positive-negative two unit zoom lens, even if chromatic aberration therein is corrected well, becomes small in the degree of freedom of aberration correction because the number of the lens units is only two, and if an attempt is made to achieve higher variable power ratio, the other aberrations than chromatic aberration will occur greatly and it will become difficult to correct this well. Particularly at the position of the focal length (zoom) intermediate of the wide angle end and the telephoto end, the image plane becomes under and therefore, if an attempt is made to maintain both of downsizing and high optical performance, the upper limit of the variable power ratio will become the order of 3.
Also, the positive-positive-negative three-unit zoom lens makes up for the drawback of the aberration correction of the two-unit zoom lens and can cope with higher variable power ratio to a certain degree. However, it is difficult to achieve higher variable power ratio and yet obtain high optical performance.
In contrast, the positive-negative-positive-positive four-unit zoom lens is suited for higher variable power and can have its back focal length made relatively long, but the optical full length thereof becomes long and this is disadvantageous for the downsizing of the entire lens system.
So, it is conceivable to reduce the number of lenses in each lens unit to thereby shorten the full length of the lens.
However, a cemented lens is used in each lens unit to correct chromatic aberration occurring in each lens unit and therefore, if the number of lenses is decreased, the element for correcting chromatic aberration will become insufficient and it will become difficult to correct chromatic aberration well within the entire variable power range.
It is the object of the present invention to provide a zoom lens in which a desired variable power ratio is secured and yet chromatic aberration can be corrected well within the entire variable power range, and an optical apparatus using the same.
In order to achieve the above object, the zoom lens of the present invention is a zoom lens which comprises, in succession from the object side, a first lens unit of negative optical power, a second lens unit of positive optical power, a third lens unit of positive optical power and a fourth lens unit of negative optical power, wherein in case of the zooming from the wide angle end to the telephoto end, the spacing between the first lens unit and the second lens unit is increased, the spacing between the second lens unit and the third lens unit is increased, and the spacing between the third lens unit and the fourth lens unit is decreased, and is characterized in that at least one of the first to fourth lens units has a diffraction optical portion.